The invention relates to an apparatus for improving the efficiency of shock waves treatment in a living body or tissue while, at the same time, reducing the harmful effects of such treatment and to a method of treating a living body or tissue with such an apparatus.
In the past two decades fragmentation of concretions inside a living body by focused shock wave from outside the body was established as a method of treatment. Recent research is also developing for the application of shock wave treatment in the field of Orthopedics and in pathological tissue ablation as well as in other different types of treatment.
The mechanisms by which focused shock waves disintegrates stones in Extracorporeal Shock Wave Lithotripsy are still not well understood. However several mechanisms for stone fragmentation have been proposed and documented in the literature.
The shock wave pulse comprises of a positive peak pressure up to about 120 Mpa, which lasts for up to about 2 microseconds followed by negative peak pressure up to about 20 MPa with about 2 to 8 microseconds duration.
It is further known in the art that the negative pressure induces transient cavitation bubbles around the focal point. Ensuing pulses cause these cavitation bubbles to collapse. When bubbles collapse adjacent to a solid surface like a stone it will take place asymmetrically leading to the formation of high speed, liquid micro jets that hit the stone surface and cause cracking and fragmentation.
Only a percentage of this micro jets, is directed to the stone while the remaining part, is consumed by the adjacent tissues leading to tissue damage.
It is also mentioned in the literature that the conditions required for fracturing stones include one or more of the following; compression and release, tension or spall and cavitation induced stress. Fragmentation involves separation of crystal layers and fracture and cleavage of crystals. The disintegration of stones occurs by the progressive initiation of cracks and their stepwise extension through the material. Brittle materials fail under compressive shock loading by initiation and growth of micro cracks from internal defects such as pores or inclusion or from material boundaries such as interfaces with organic or fibrous material or grain boundaries.
With repeated pulses the micro cracks grow on a prospective spall plane and they coalesce on reaching a critical length creating a fragment. Under pressure, the micro cracks grow in the axial plane; i.e. the failure is in the direction of maximum applied compression that is the direction of shock wave propagation. On the other hand, under tension micro cracks grow on a plane perpendicular to the direction of applied tension i.e. perpendicular to the direction of the shock wave propagation.
The present invention is based upon the discovery that the collapse of the cavitation bubbles can be controlled by the timing and direction of additional shock waves. Hence the use of one or more other shock wave sources to generate another shock wave propagating at an appropriate angle from the direction of propagation of the primary shock wave will enhance the treatment effect of shockwave and would abolish or minimize the tissue damage out side the focal area. The value of the angle between these shock wave sources varies according to various factors such as the type of treatment and the level of energy. The timing of generation of shock waves from these sources could be instantaneous or with a delay period between each of up to about 100 millisecond. This delay varies according to various factors such as the type of treatment and level of energy.